1. Field of the Invention
This invention relates to a process for producing substantially crystalline graphitic carbon nanofibers comprised of graphite sheets. The graphite sheets are substantially perpendicular to the longitudinal axis of the carbon nanofiber. These carbon nanofibers are produced by contacting an iron:copper bimetallic bulk catalyst with a mixture of carbon monoxide and hydrogen at temperatures from about 550xc2x0 C. to about 670xc2x0 C. for an effective amount of time.
2. Description of Related Art
Nanostructure materials, particularly carbon nanostructure materials, are quickly gaining importance for various potential commercial applications. Such applications include their use to store molecular hydrogen, serve as catalyst supports, as reinforcing components for polymeric composites and to be useful in various batteries. Carbon nanostructure materials are typically prepared from the decomposition of carbon-containing gases over selected catalytic metal surfaces at temperatures ranging from about 500xc2x0 to about 1,200xc2x0 C.
For example, U.S. Pat. Nos. 5,149,584 and 5,618,875, to Baker et al., teach carbon nanofibers as reinforcing components in polymer reinforced composites. The carbon nanofibers can either be used as is or as part of a structure comprised of carbon fibers having carbon nanofibers grown therefrom. The examples of these patents show the preparation of various carbon nanostructures by the decomposition of a mixture of ethylene and hydrogen in the presence of metal catalysts such as iron, nickel, a nickel:copper alloy, an iron:copper alloy, etc. Also,U.S. Pat. No. 5,413,866, to Baker et al., teaches carbon nanostructures characterized as having: (i) a surface area from about 50 m2/g to 800 m2/g; (ii) an electrical resistivity from about 0.3 xcexcohmxc2x7m to 0.8 xcexcohmxc2x7m; (iii) a crystallinity from about 5% to about 100%; (iv) a length from about 1 xcexcm to about 100 xcexcm; and (v) a shape that is selected from the group consisting of branched, spiral, and helical. These carbon nanostructures are taught as being prepared by depositing a catalyst containing at least one Group IB metal, and at least one other metal, on a suitable refractory support and then subjecting the catalyst-treated support to a carbon-containing gas at a temperature from the decomposition temperature of the carbon-containing gas to the deactivation temperature of the catalyst.
U.S. Pat. No. 5,458,784, also to Baker et al., teaches the use of the carbon nanostructures of U.S. Pat. No. 5,413,866 for removing contaminants from aqueous and gaseous steams; and U.S. Pat. No. 5,653,951, to Rodriguez et al., discloses and claims that molecular hydrogen can be stored in layered nanostructure materials having specific distances between layers. The examples of these patents teach the aforementioned preparation methods as well as the decomposition of a mixture of carbon monoxide and hydrogen in the presence of an iron powder catalyst at 600xc2x0 C. All of the above referenced U.S. patents are incorporated herein by reference.
While various carbon nanostructures and their uses are taught in the art, there is still a need for improvements before such nanostructure materials can reach their full commercial and technical potential. For example, while the art broadly discloses carbon nanostructures having crystallinities from about 5 to 95%, it has heretofore not been possible to produce carbon nanostructures with crystallinities greater than about 95%.
In accordance with the present invention, there is provided a substantially crystalline graphitic carbon nanofibers comprised of graphite sheets that are substantially perpendicular to its longitudinal axis of the nanofibers, wherein the distance between graphite sheets is from about 0.335 nm to about 0.67 nm, and having a crystallinity greater than about 95%.
In a preferred embodiment, the distance between the graphite sheets is from about 0.335 and 0.40 nm.
Also in accordance with the present invention, there is provided a process of producing substantially crystalline graphitic carbon nanofibers which process comprises reacting a mixture of CO/H2 in the presence of a powder Fe:Cu bimetallic catalyst for an effective amount of time at a temperature from about 550xc2x0 C. to about 670xc2x0 C.
In a preferred embodiment, the ratio of Fe to Cu is from about 5:95 to about 95:5 and the ratio of CO to H2 is from about 95:5 to about 5:95, preferably from about 80:20 to about 20:80.
The carbon nanofibers of the present invention possess a novel structure in which graphite sheets, constituting the material, are aligned in a direction that is substantially perpendicular to the growth axis (longitudinal axis) of the nanofiber. The carbon nanfibers are sometimes referred to herein as xe2x80x9cplateletxe2x80x9d nanofibers. In addition, the nanofibers have a unique set of properties, which include: (i) a nitrogen surface area from about 40 to 120 m2/g; (ii) an electrical resistivity of 0.4 ohmxc2x7cm to 0.1 ohmxc2x7cm; (iii) a crystallinity from about 95% to 100%; and (iv) a spacing between adjacent graphite sheets of 0.335 nm to about 1.1 nm, preferably from about 0.335 nm to about 0.67 nm, and more preferably from about 0.335 to about 0.40 nm.
The catalysts used to prepare the carbon nanofibers of the present invention are iron:copper bulk bimetallic catalysts in powder form. It is well established that the ferromagnetic metals, iron, cobalt and nickel, are active catalysts for the growth of carbon nanofibers during decomposition of certain hydrocarbons or carbon monoxide. Efforts are now being directed at modifying the catalytic behavior of these metals, with respect to nanofiber growth, by introducing other metals and non-metals into the system.
In this respect, copper is an enigma, appearing to be relatively inert towards carbon deposition during the CO/H2 reaction. Thus, it is unexpected that the combination of Cu with Fe has such a dramatic effect on carbon nanofiber growth in the CO/H2 system.
The average powder particle size of the metal catalyst will range from about 0.5 nanometer to about 5 micrometer, preferably from about 2.5 nanometer to about 1 micrometer. The ratio of the two metals can be any effective ratio that will produce substantially crystalline carbon nanofibers in which the graphite sheets are substantially perpendicular to the longitudinal axis of the nanofiber, at temperatures from about 550xc2x0 C. to about 670xc2x0 C. in the presence of a mixture of CO/H2. The ratio of iron:copper will, typically, be from about 5:95 to about 95:5, preferably from about 3:7 to about 7:3; and more preferably from about 6:4 to about 7:3. The bimetallic catalyst can be prepared by any suitable technique. One preferred technique is by co-precipitation of aqueous solutions containing soluble salts of the two metals. Preferred salts include the nitrates, sulfates, and chlorides of iron and copper, particularly iron nitrate and copper nitrate. The resulting precipitates are dried and calcined to convert the salts to the mixed metal oxides. The calcined metal powders are then reduced at an effective temperature and for an effective time.
The iron:copper catalyst powders used in the present invention are prepared by the co-precipitation of aqueous solutions containing appropriate amounts of nickel and copper nitrate using ammonium bicarbonate. The precipitates were dried overnight at the 110xc2x0 C. before being calcined in air at 400xc2x0 C. to convert the carbonates into mixed metal oxides. The calcined powders were then reduced in hydrogen for 20 hours at 400xc2x0 C. Following this treatment, the reduced catalyst was cooled to room temperature in a helium environment before being passivated in a 2% oxygen/helium mixture for 1 hour at about room temperature (24xc2x0 C.).
Gas flow reactor experiments were carried out in a horizontal quartz tube (40 mm i.d. and 90 cm ong) contained in a Linberg tube furnace, at temperatures over the range of about 450xc2x0 C. to 700xc2x0 C. Gas flow rates to the reactor were regulated by MKS mass flow controllers. In a typical experiment, 50 mg of the given catalyst powder was dispersed in a substantially uniform manner along the base of a ceramic boat, which was subsequently placed at the center of the reactor tube. After reduction of the sample at 600xc2x0 C. for 2 hours, the system was flushed with helium and brought to the desired temperature level before being reacted with in the CO/H2 mixture for a period of 2 hours. The total amount of solid carbon formed in any given experiment was determined at the completion of the reaction by weight difference. The composition of the gas phase was measured at regular intervals by taking samples of the inlet and outlet streams which were then analyzed by gas chromatography using a 30 m megabore (CS-Q) capillary column in a Varian 3400 GC unit. Carbon and hydrogen atom balances, in combination with the relative concentrations of the respective components, were applied to obtain the various product yields. In order to obtain reproducible carbon deposition data, it was necessary to follow an identical protocol for each experiment.
The structural details of the carbon materials, resulting from the interaction of the CO/H2 mixtures with the various powdered bimetallic catalysts, were examined in a JEOL 2000 EX II transmission electron microscope that was fitted with a high resolution pole piece capable of providing a lattice resolution of 0.18 nm. Temperature programmed oxidation studies (TPO) of the various carbon materials were carried out in a Cahn 2000 microbalance in the presence of a CO2/Ar (1:1) mixture at a heating rate of 5xc2x0/min up to a maximum of a given carbon deposit from a comparison the oxidation profile with those of two standard materials, amorphous carbon and single crystal graphite when treated under the same conditions.
It is known that, carbon nanostructures can be prepared by reacting a catalyst in a heating zone with the vapor of a suitable carbon-containing compound. While the art teaches a wide variety of carbon-containing compounds as being suitable, the inventors hereof have found that only a mixture of CO and H2 will yield carbon nanofibers with unexpected high crystallinities. That is, crystallinities greater than about 95%, preferably greater than 97%, more preferably greater than 98%, and most preferably substantially 100%.
After the nanofibers are grown, it may be desirable to treat them with an aqueous solution of an inorganic acid, such as a mineral acid, to remove any excess catalyst particles. Non-limiting examples of suitable mineral acids include sulfuric acid, nitric acid and hydrochloric acid. Preferred is sulfuric acid.
It is within the scope of this invention to increase the spacing between the graphite sheets by any suitable means, such as by intercalation. Intercalation involves incorporating an appropriate intercalation compound between platelets. Intercalation compounds suitable for graphite structures are comprehensively discussed in Applications of Graphite Intercalation Compounds, by M. Inagaki, Journal of Material Research, Vol 4, No.6, Nov/Dec 1989, which is incorporated herein by reference. The preferred intercalation compounds for use with the nanofibers of the present invention are alkali and alkaline-earth metals. The limit to which the platelet spacing will be increased for purposes of the present invention will be that point wherein the carbon nanofibers no longer can be characterized as graphitic. That is, the spacing can become so large the the carbon now has properties more like amorphorous carbon instead of graphite. It is important for the practice of the present invention that the carbon nanofibers maintain the basal plane structure representative of graphite.
A major advantage of the graphite nanofibers of the present invention, over other graphitic materials, is their flexibility with regard to modification of surface chemistry. For example, the edge regions of the nanofibers can be made either basic (introduction of NH4+ groups) or acidic (addition of COOHxe2x88x92 groups) by use of appropriate methods. Furthermore, the presence of oxygenated groups (hydroxyl, peroxide, ether, keto or aldehyde), that are neither acidic nor basic in nature, can impart polarity to the graphite structure. Polar groups will promote the interaction of carbon edge atoms with other polar groups such as water. As a consequence, the interaction of graphitic materials with aqueous solutions can be greatly enhanced due to the presence of acid, basic or neutral functionality.
The distribution of polar groups in active carbon (non-graphitic) occurs in a random fashion, whereas in materials such as the graphite nanofibers of the present invention, such sites are always located at the edges of the graphene layers. Addition of oxygenated groups can be achieved by selected oxidation treatments including treatment in peroxides, nitric acid, potassium permanganate, etc. Polar sites can also be eliminated by reduction, out-gassing in vacuum at 1000xc2x0 C. or treatment in hydrazine at 80xc2x0 C. Following this procedure, the graphite nanofiber will become hydrophobic. Theodoridou and coworkers, (Met. 14, 125 (1986)), demonstrated that very efficient surface oxidation of carbon fibers can be achieved by d.c. oxidation or repetitive anodic oxidation and cathodic reduction of the material in acidic, alkaline or neutral aqueous media. It was believed that this method had the advantage over other procedures in that thick layers of surface oxides could be produced without damaging the fiber structure. These workers also capitalized on the conductive properties of graphitized carbon fibers to introduce various noble metals onto such materials via the use of electrochemical procedures. The possibility of controlling the functionality of the graphite surface could have a direct impact on both the chemistry of the supported metal particles and their morphological characteristics.